Although Zoega lists <kné> as m(asculine), I think this must be a misprint, because Cleasby/Vigfusson lists it as n(euter), and Gordon (An Introduction to Old Norse) uses it as a paradigm for a stong declension a-stem neuter noun, giving <kné> as the accusative plural form.
> Ok fell á kné fyrir honum ok lagði sik í hans vald ok allt
And fell to her knee before him and placed herself in his
> sitt vald ok land.
> And fell on (her) knees (grammatically could be sg or
> plural, but one usually falls on one’s knees and goes down
> on one knee) before him and placed herself in his power
> and all her dominion (sg vald, Z2) and land.
power, and all her authority and land.
<Kné> is masculine, so <kné> here can only be accusative
singular.